The present invention relates to a battery device (hereinafter, referred to as "battery pack") including a circuit required to be monitored for a battery state such as a voltage or a charge/discharge current, a battery state monitoring circuit for monitoring the circuit, an external connection terminal for connection to a device outside of the battery device, a switch element, a secondary battery and a sense resistor in a battery pack for a secondary battery.
As a conventional battery state monitoring circuit, there has been known a device shown in a circuit block diagram of FIG. 2. For example, Japanese Patent Application Laid-open No. Hei 9-312172, entitled "Battery pack, charger and charging system as well as charging method", discloses a structure of this type. This is directed to a battery pack which is commonly called a "smart battery system" or the like. That is, this is a battery pack 100a having a function of monitoring a voltage and charge/discharge current of the secondary battery by use of a microcomputer 5a, and communicating with a load such as a charger 17 or a computer through terminals 101, 102.
Employing the battery pack 100a thus structured, it is possible to recognize a battery state by communication with the charger 17, the microcomputer within a personal computer, a load 16 or the like. The use of this information allows an indication of the residual amount of the battery, a suspension of battery charge, etc., to be conducted accurately. In the case where a lithium ion battery is used as a secondary battery, since a self-protecting action is not provided, unlike that of a nickel cadmium battery, an over-charge protecting circuit is required. That is, the lithium ion battery is provided with a circuit for detection of a battery voltage and a switch element for suspending a charging operation from the outside.
In the battery pack 100a thus structured, a microcomputer 5a is used. The battery pack 100a is also equipped with an amplifier 3a for monitoring a battery voltage and a sense resistor 10 and an amplifier 3b for monitoring a charge/discharge current. To the microcomputer 5a is supplied electric signals from a battery voltage monitor circuit 20a and amplifiers 3a , 3b. Since the microcomputer 5a has a calculating function and an A/D converter so that it can calculate the voltage and the capacity of the secondary batteries 6 to 8 from the above-described signals, it can monitor a battery state. This makes it possible for the microcomputer 5a to control the on/off state of the switch elements 11a and 11b, and therefore the microcomputer 5a provides safety with respect to over-charging of the battery pack in which a lithium ion battery is used.
It is necessary for a constant voltage to be provided as a power supply of the microcomputer 5a which is a structurally important part. For example, a voltage of 3.3 V or 5.0 V is a normal value. If the supply voltage applied to the microcomputer 5a is unstable, the detection accuracy of the battery voltage, etc., are degraded. In the worst case, there generally occurs a phenomenon called "runaway" of the microcomputer 5a. This creates an environment in which the microcomputer 5a is not controlled at all, with the result that the safety of the battery pack 100a is not assured at all. In addition, it is desirable for a constant voltage to be applied to the battery voltage monitoring circuit 20a and the amplifiers 3a, 3b for their safe operation.
Because the power supply for the battery state monitoring circuit 18a within the battery pack 100a is made up of the secondary batteries 6 to 8, the voltage is varied according to the load state. In the case where the battery pack 100a is discharged to the load, the supply voltage becomes low, whereas in the case where the battery pack 100a is charged, the supply voltage becomes high. In order to apply a constant voltage to the microcomputer 5a and the amplifier 3a,3b equipped within the battery pack 100a, a voltage regulator is disposed within the battery pack. The voltage regulator serves to maintain a constant output voltage constant even if the supply voltage is varied.